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FORK(3P)                   POSIX Programmer's Manual                  FORK(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       fork - create a new process
SYNOPSIS
       #include <unistd.h>
       pid_t fork(void);

DESCRIPTION
       The fork() function shall create a new process. The new process  (child
       process) shall be an exact copy of the calling process (parent process)
       except as detailed below:
        * The child process shall have a unique process ID.
        * The child process ID also shall not match any active  process  group
          ID.
        * The  child  process  shall have a different parent process ID, which
          shall be the process ID of the calling process.
        * The child process shall have its  own  copy  of  the  parent's  file
          descriptors.   Each  of  the child's file descriptors shall refer to
          the same open file description with the corresponding file  descrip-
          tor of the parent.
        * The  child  process  shall  have  its  own copy of the parent's open
          directory streams. Each open directory stream in the  child  process
          may share directory stream positioning with the corresponding direc-
          tory stream of the parent.
        * The child process shall have its own copy of  the  parent's  message
          catalog descriptors.
        * The  child  process' values of tms_utime, tms_stime, tms_cutime, and
          tms_cstime shall be set to 0.
        * The time left until an alarm clock signal shall be  reset  to  zero,
          and the alarm, if any, shall be canceled; see alarm().
        * All semadj values shall be cleared.
        * File  locks  set by the parent process shall not be inherited by the
          child process.
        * The set of signals pending for the child process shall  be  initial-
          ized to the empty set.
        * Interval timers shall be reset in the child process.
        * Any  semaphores  that  are  open in the parent process shall also be
          open in the child process.
        * The child process shall not inherit any address space  memory  locks
          established  by  the  parent  process  via  calls  to  mlockall() or
          mlock().
        * Memory mappings created in the parent shall be retained in the child
          process.  MAP_PRIVATE  mappings inherited from the parent shall also
          be MAP_PRIVATE mappings in the child, and any modifications  to  the
          data  in  these  mappings made by the parent prior to calling fork()
          shall be visible to the child. Any  modifications  to  the  data  in
          MAP_PRIVATE  mappings  made by the parent after fork() returns shall
          be visible only to the parent. Modifications to the data in MAP_PRI-
          VATE mappings made by the child shall be visible only to the child.
        * For  the  SCHED_FIFO  and  SCHED_RR  scheduling  policies, the child
          process shall inherit the policy and priority settings of the parent
          process during a fork() function. For other scheduling policies, the
          policy and priority settings on fork() are implementation-defined.
        * Per-process timers created by the parent shall not be  inherited  by
          the child process.
        * The  child  process  shall  have  its  own copy of the message queue
          descriptors of the parent. Each of the message  descriptors  of  the
          child  shall refer to the same open message queue description as the
          corresponding message descriptor of the parent.
        * No asynchronous input or asynchronous  output  operations  shall  be
          inherited by the child process.
        * A process shall be created with a single thread. If a multi-threaded
          process calls fork(), the new process shall contain a replica of the
          calling  thread and its entire address space, possibly including the
          states of mutexes  and  other  resources.   Consequently,  to  avoid
          errors,  the child process may only execute async-signal-safe opera-
          tions until such time as one of the exec functions is called.   Fork
          handlers  may  be established by means of the pthread_atfork() func-
          tion in order  to  maintain  application  invariants  across  fork()
          calls.
       When  the application calls fork() from a signal handler and any of the
       fork handlers registered by pthread_atfork() calls a function  that  is
       not asynch-signal-safe, the behavior is undefined.
        * If the Trace option and the Trace Inherit option are both supported:
       If  the calling process was being traced in a trace stream that had its
       inheritance policy set  to  POSIX_TRACE_INHERITED,  the  child  process
       shall  be  traced  into  that trace stream, and the child process shall
       inherit the parent's mapping of trace event names to trace  event  type
       identifiers. If the trace stream in which the calling process was being
       traced had its inheritance policy set  to  POSIX_TRACE_CLOSE_FOR_CHILD,
       the  child  process  shall  not  be  traced into that trace stream. The
       inheritance policy is set by a call to  the  posix_trace_attr_setinher-
       ited() function.
        * If  the  Trace  option is supported, but the Trace Inherit option is
          not supported:
       The child process shall not be traced into any of the trace streams  of
       its parent process.
        * If  the Trace option is supported, the child process of a trace con-
          troller process shall not control the trace  streams  controlled  by
          its parent process.
        * The  initial  value of the CPU-time clock of the child process shall
          be set to zero.
        * The initial value of the CPU-time clock of the single thread of  the
          child process shall be set to zero.
       All other process characteristics defined by IEEE Std 1003.1-2001 shall
       be the same in the parent and  child  processes.   The  inheritance  of
       process characteristics not defined by IEEE Std 1003.1-2001 is unspeci-
       fied by IEEE Std 1003.1-2001.
       After fork(), both the parent and the child processes shall be  capable
       of executing independently before either one terminates.
RETURN VALUE
       Upon  successful completion, fork() shall return 0 to the child process
       and shall return the process ID of the  child  process  to  the  parent
       process. Both processes shall continue to execute from the fork() func-
       tion. Otherwise, -1 shall be returned to the parent process,  no  child
       process shall be created, and errno shall be set to indicate the error.
ERRORS
       The fork() function shall fail if:
       EAGAIN The  system  lacked  the  necessary  resources to create another
              process, or the system-imposed limit on the total number of pro-
              cesses   under   execution  system-wide  or  by  a  single  user
              {CHILD_MAX} would be exceeded.

       The fork() function may fail if:
       ENOMEM Insufficient storage space is available.

       The following sections are informative.
EXAMPLES
       None.
APPLICATION USAGE
       None.
RATIONALE
       Many historical implementations have timing windows where a signal sent
       to  a  process group (for example, an interactive SIGINT) just prior to
       or during execution of fork() is delivered to the parent following  the
       fork()  but not to the child because the fork() code clears the child's
       set of pending signals.  This volume of IEEE Std 1003.1-2001  does  not
       require,  or  even  permit,  this behavior. However, it is pragmatic to
       expect that problems of this nature may continue to exist in  implemen-
       tations  that  appear to conform to this volume of IEEE Std 1003.1-2001
       and pass available verification suites.  This behavior is only a conse-
       quence  of the implementation failing to make the interval between sig-
       nal generation and delivery totally invisible. From  the  application's
       perspective,  a fork() call should appear atomic. A signal that is gen-
       erated prior to the fork() should be delivered prior to the fork().   A
       signal  sent  to the process group after the fork() should be delivered
       to both parent and child. The implementation  may  actually  initialize
       internal  data  structures  corresponding to the child's set of pending
       signals to include signals sent to the process group during the fork().
       Since  the  fork()  call  can be considered as atomic from the applica-
       tion's perspective, the set would be initialized as empty and such sig-
       nals would have arrived after the fork(); see also <signal.h>.
       One  approach  that has been suggested to address the problem of signal
       inheritance across fork() is to add an [EINTR] error,  which  would  be
       returned  when  a  signal  is  detected  during the call. While this is
       preferable to losing signals, it was not considered  an  optimal  solu-
       tion.  Although  it  is not recommended for this purpose, such an error
       would be an allowable extension for an implementation.
       The [ENOMEM] error value is reserved  for  those  implementations  that
       detect  and distinguish such a condition. This condition occurs when an
       implementation detects that there is not enough memory  to  create  the
       process. This is intended to be returned when [EAGAIN] is inappropriate
       because there can never be enough memory (either primary  or  secondary
       storage) to perform the operation.  Since fork() duplicates an existing
       process, this must be a condition where there is sufficient memory  for
       one  such  process,  but  not  for two. Many historical implementations
       actually return [ENOMEM] due to temporary lack of memory, a  case  that
       is  not generally distinct from [EAGAIN] from the perspective of a con-
       forming application.
       Part of the reason for including the optional error [ENOMEM] is because
       the SVID specifies it and it should be reserved for the error condition
       specified there. The condition is not applicable  on  many  implementa-
       tions.
       IEEE Std 1003.1-1988  neglected  to require concurrent execution of the
       parent and child of fork(). A system that single-threads processes  was
       clearly not intended and is considered an unacceptable "toy implementa-
       tion" of this volume of IEEE Std 1003.1-2001. The only objection antic-
       ipated to the phrase "executing independently" is testability, but this
       assertion should be testable. Such tests require that both  the  parent
       and  child  can  block  on  a detectable action of the other, such as a
       write to a pipe or a signal. An interactive exchange  of  such  actions
       should be possible for the system to conform to the intent of this vol-
       ume of IEEE Std 1003.1-2001.
       The [EAGAIN] error exists to warn applications that  such  a  condition
       might  occur.  Whether  it  occurs or not is not in any practical sense
       under the control of the application because the condition is usually a
       consequence  of  the user's use of the system, not of the application's
       code. Thus, no application can or should rely upon its occurrence under
       any  circumstances,  nor  should the exact semantics of what concept of
       "user" is used be of concern  to  the  application  writer.  Validation
       writers should be cognizant of this limitation.
       There  are two reasons why POSIX programmers call fork(). One reason is
       to create a new thread of control within the same  program  (which  was
       originally only possible in POSIX by creating a new process); the other
       is to create a new process running a different program. In  the  latter
       case,  the call to fork() is soon followed by a call to one of the exec
       functions.
       The general problem with making fork() work in a  multi-threaded  world
       is  what to do with all of the threads. There are two alternatives. One
       is to copy all of the threads into the new process.   This  causes  the
       programmer or implementation to deal with threads that are suspended on
       system calls or that might be about to execute system calls that should
       not  be  executed  in the new process. The other alternative is to copy
       only the thread that calls fork(). This creates the difficulty that the
       state  of process-local resources is usually held in process memory. If
       a thread that is not calling fork() holds a resource, that resource  is
       never  released in the child process because the thread whose job it is
       to release the resource does not exist in the child process.
       When a programmer  is  writing  a  multi-threaded  program,  the  first
       described  use  of fork(), creating new threads in the same program, is
       provided by the pthread_create() function.  The fork() function is thus
       used  only  to  run  new programs, and the effects of calling functions
       that require certain resources between the call to fork() and the  call
       to an exec function are undefined.
       The  addition  of the forkall() function to the standard was considered
       and rejected. The forkall() function lets all the threads in the parent
       be  duplicated  in  the child. This essentially duplicates the state of
       the parent in the child. This allows threads in the child  to  continue
       processing  and  allows  locks  and  the  state to be preserved without
       explicit pthread_atfork() code. The calling process has to ensure  that
       the  threads  processing  state  that  is shared between the parent and
       child (that is, file descriptors or MAP_SHARED memory) behaves properly
       after  forkall(). For example, if a thread is reading a file descriptor
       in the parent when forkall() is called, then two threads  (one  in  the
       parent  and one in the child) are reading the file descriptor after the
       forkall(). If this is not desired behavior, the parent process  has  to
       synchronize with such threads before calling forkall().
       While  the fork() function is async-signal-safe, there is no way for an
       implementation to determine whether the fork  handlers  established  by
       pthread_atfork()  are async-signal-safe.  The fork handlers may attempt
       to execute portions of the implementation that  are  not  async-signal-
       safe,  such  as those that are protected by mutexes, leading to a dead-
       lock condition. It is therefore undefined for the fork handlers to exe-
       cute  functions  that  are  not async-signal-safe when fork() is called
       from a signal handler.
       When forkall() is called, threads, other than the calling thread,  that
       are  in  functions that can return with an [EINTR] error may have those
       functions return [EINTR] if the implementation cannot ensure  that  the
       function  behaves  correctly  in  the  parent and child. In particular,
       pthread_cond_wait() and  pthread_cond_timedwait()  need  to  return  in
       order to ensure that the condition has not changed. These functions can
       be awakened by  a  spurious  condition  wakeup  rather  than  returning
       [EINTR].
FUTURE DIRECTIONS
       None.
SEE ALSO
       alarm(),      exec(),     fcntl(),     posix_trace_attr_getinherited(),
       posix_trace_trid_eventid_open(), pthread_atfork(),  semop(),  signal(),
       times(),   the   Base   Definitions   volume  of  IEEE Std 1003.1-2001,
       <sys/types.h>, <unistd.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                             FORK(3P)